Piezoelectric precision positioning device

ABSTRACT

A piezoelectric precision positioning device for moving an object in three coordinate directions is described, comprising 
     (a) an at least approximately plane-parallel cut piezoelectric disk (5), 
     (b) an electrode arrangement (6, 7, 8, 17), provided with electrical connections (9, 10, 11, 18), on the surfaces parallel to each other, with which three independently controllable electrical fields in the piezoelectric disk (5) can be adjusted, 
     (c) a fork-shaped clamping device (1, 2) which embraces the piezoelectric disk (5), in an electrically insulated manner with respect to the electrode arrangement (6, 7, 8, 17), at at least three pressure points (13, 14, 15, 16), which are situated symmetrically with respect to each other, on the circumferential surface such that 
     (d) one of the electrodes (8) is situated in the open part of the fork-shaped clamping device (1, 2), in which case 
     (e) the object (19) to be positioned is arranged in an electrically insulated manner on an electrode surface (17) in the field region of this electrode (8).

BACKGROUND OF THE INVENTION

The invention relates to a piezoelectric precision positioning devicefor moving an object in three coordinate directions and a method forcontrolling the precision positioning device.

As precision positioning in the context of the invention, a positioningaccuracy of better than 10⁻¹⁰ m is understood, i.e. a movement in theregion of atomic dimensions. Devices of this type form e.g. the basisfor the examination of surface topography using the tunnel effect. Inthis case, the precision positioning device moves a pointed scanningneedle at a specified distance in a raster pattern across the surface tobe investigated, in which process e.g. the tunnel current is used toregulate the distance and the controlled variable is represented as afunction of the position signal of the scanning tip (scanning tunnelmicroscopy).

The precision positioning device must fulfil two requirements inparticular. It must be particularly stable mechanically in order torender the system insensitive to vibrations in the environment. Theresonance frequency of the device should be as high as possible in orderto be able to achieve as high a scanning raster velocity as possiblewhich, in addition to higher spatial resolution, also makes a bettertime resolution possible in the observation of dynamic processes.

The devices of this type known hitherto use piezoelectric components aspositioning elements. In the case of the piezoelectric tripod describedin Phys. Rev. Lett., volume 49, No. 1 (1982), pages 57-61, e.g. threestrips of a piezoelectric ceramic situated at right angles to each otherare combined in the form of a pyramid. The tip of the pyramid can bemoved by altering the length of the strips in the three spatialdirections. A disadvantage of this arrangement is its relatively largemass which results in a low resonance frequency and, consequently, in anincreased susceptibility in relation to thermal drift phenomena.

In another form of construction known from Rev. Sci. Instrum., vol. 56,No. 8 (August 1985), pages 1573-1576, very small piezoelectric ceramiccubes and metal cubes are glued onto each other in checker-boardfashion. Although this positioning device has a smaller mass, a largeproportion of the total mass is formed from material which is notpiezoelectrically active. The interposed metal cubes also reduce theresonance frequency here.

Common to both forms of construction is the fact that they consist of atleast three different pieces of a piezoelectric ceramic which are joinedto each other by gluing, screwing, clamping or the like. This introducesmechanical weak points which, under cyclic thermal conditions, areseverely loaded mechanically owing to the different thermal coefficientsof expansion of the materials which are rigidly joined together.

The invention was therefore based on the object of providing a precisionpositioning device which exhibits a low vibration sensitivity, has ahigh resonance frequency which is substantially insensitive totemperature, and, in particular, can also be used in the low-temperaturerange, and which makes possible a flat constructional form and as simplea construction as possible.

SUMMARY OF THE INVENTION

An outstanding feature of the device according to the invention is thesystematic minimization of parasitic, piezoelectrically inactive masseswith, at the same time, an extremely low total mass of the actual drivebody. The latter consists of a single, inherently homogeneouspiezoelectric component. The clamping device, which is static relativeto the drive body, may be manufactured from a material whose thermalcoefficient of expansion is matched to that of the drive body. Inaddition, in controlling the drive body, only a negligible relativemovement takes place between the drive body and the clamping points.Insofar as distance variations occur between the clamping points, theyare absorbed by the elasticity of the fork jaws of the clamping device.

Exemplary embodiments of the device according to the invention are showndiagrammatically in the drawing. Their mode of operation is describedbelow by reference to the figures. In detail:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a view from below of the precision positioning device,

FIG. 2 shows a plan view of the precision positioning device,

FIG. 3 shows a front view of the precision positioning device,

FIG. 4 shows a diagrammatic representation of the movements in

(a,b) the tangential,

(c) the axial and

(d) the radial direction,

FIG. 5 shows another possibility for the arrangement of the electrodes,

FIG. 6 shows a block circuit diagram of the control system.

DETAILED DESCRIPTION OF THE INVENTION

The precision positioning device shown in FIG. 1 contains a fork-shapedclamping device composed of two jaws 1, 2. The two jaws 1, 2 are heldtogether by a bearing screw 4 and can be adjusted with respect to eachother as a gripping device by a clamping screw 3. In a simplerrefinement, the jaws may be rigidly joined together in the region of theclamping screw, in which case bracing against the spring force of thejaws can then take place in the region of the screw 4. The clampingdevice may be manufactured from brass. However, a ceramic orglass-ceramic material may be selected if this is more expedient e.g.because of the thermal properties.

The actual drive body, which is composed of a round, approximatelyplane-parallel piezoelectric disk 5, is arranged inside the jaws 1, 2.The underside of the piezoelectric disk 5 shown in FIG. 1 is occupied bythree two-dimensional electrodes 6, 7, 8 to which an electrical controlvoltage can be applied in each case via electrical connections 9, 10,11. The line of separation 12 between the electrode pads has the shapeof a Y. The areas of the electrodes 6, 7 are at least approximatelyequally large, while that of the electrode 8 is very small in comparisontherewith. Its maximum length and width should not be larger than thethickness of the disk 5.

The piezoelectric disk 5 is held in the pressure points 13, 14 of theouter parts of the jaws 1, 2 and, in the inner region of the jaws, is incontact with two points 15, 16 which are close together and which mayalso be considered as a single pressure point. The jaw length is sochosen in relation to the pressure points 13, 14 that, although thecenter point of the disk 5 lies within the fork area of the clampingdevice terminated by the connecting line between the pressure points 13,14, it lies in the vicinity of said connecting line. This measureachieves the result that, on the one hand, the disk 5 cannot be pushedforwards out of the clamping device, and on the other hand, as large aregion of the disk area as possible lies outside the disk area enclosedby the pressure points 13, 14, 15, 16. It has proved expedient for thepressure points 13, 14 to be so chosen that at least 1/3 of the diskcircumference bridges the free part of the fork jaws 1, 2.

The orientation of the electrode areas is so chosen that the line ofseparation between the electrodes 6, 7 lies on the line of symmetry ofthe clamping device and the area lying in the V of the Y lies outside ofthe disk area enclosed by the pressure points 13, 14, 15, 16.

From the plan view shown in FIG. 2 it can be seen that the top side ofthe piezoelectric disk 5 is occupied, over the entire surface, by afurther electrode 17 to which a certain electrical potential canlikewise be applied via an electrical connection 18. When interactingwith the electrodes 6, 7, 8, this may also be e.g. the ground potential.All the electrodes are so deposited on the piezoelectric disk 5 thatthey are adequately insulated electrically from each other and withrespect to the clamping device. Only in the case of an electrodeprovided as ground electrode is it possible for it to be expedient toelectrically connect it directly to the clamping device if the latter iscomposed of electrically conducting material.

A scanning needle 19 is arranged on the electrode 17 as the object to bepositioned. For use in a tunneling microscope, this may be e.g. atungsten tip. The scanning needle 19 may, for example, be mounted withan electrically insulating glue on the electrode area 17 so that acurrent flow picked up by the scanning needle 19 can be conducted awayvia an electrical connection 20 for measurement purposes. The mountingpoint of the scanning needle 19 lies in the electrical field region ofthe electrode 8. The tip of the scanning needle 19 reaches slightlybeyond the edge of the piezoelectric disk in an extension of the line ofsymmetry already mentioned of the clamping device.

The front view in FIG. 3 makes clear how the piezoelectric disk 5 isheld in the jaws 1, 2 of the clamping device. As an amplification ofFIGS. 1 and 2, the clamping device is in this case mounted on a baseplate 21. This may be e.g. a small glass plate. The precisionpositioning device consequently itself becomes an object capable ofbeing positioned which may be displaced e.g. by the electricallycontrollable drive device described in the older application No. P3,614,996.9.

The movements, described as tangential, axial and radial, of the tip ofthe scanning needle 19 will be explained by reference to the diagramsshown in FIG. 4 which are considerably simplified and considerablyexaggerated in relation to the volume displacements in the piezoelectricdisk 5. The diagrams in FIGS. 4a, b, c are derived from the front viewin FIG. 3, while FIG. 4d corresponds to a side view of FIG. 2.

It is known that, as a result of an electrical voltage applied to theelectrodes of a piezoelectric disk, a contraction or dilation of thethickness of the disk can be produced which, because the volume of thedisk is constant, is associated, as a rule, with a change in diameter.In the older application No. P3,614,996.9, however, an electrodearrangement with a special voltage supply was specified for which volumedisplacements develop inside the disk which keep the diameter constant.Use is also made of this effect in the case of the present object ofinvention in relation to positioning in the tangential direction, i.e. amovement of the scanning needle 19 in a direction parallel to theconnecting line between the pressure points 13, 14.

A movement in the tangential direction is accordingly achieved if thedifference in potentials is altered with the sum of the potentials onthe electrodes 6, 7 remaining constant. In the region of the field ofthese electrodes, the thickness of the disk is small compared with thelength or the width of the disk so that the main effect of the change involtage produces a stretching or compression of the piezoelectricregions situated beneath the electrodes in the direction of the plane ofthe disk. Since the effects in the region of the two electrodes aredirected opposite to each other because of the special control voltages(constancy of the sum of the potentials), the center line of thepiezoelectric disk, which has the same direction as the line ofseparation between the electrodes 6, 7, is moved sideways. The scanningneedle 19 mounted on this line therefore likewise moves in thisdirection described as tangential.

FIG. 4a illustrates the case where the potential on the electrode 6produces a maximum dilation and that on the electrode 7 produces amaximum contraction. Since the outer boundary of the disk 5 cannot giveway because of the clamping, the necessary equalization of volume takesplace in the region of the center line. The scanning needle follows thetraveling volume to the left. FIG. 4b shows the conditions if thepotentials are interchanged. The scanning needle is then displaced tothe right. The maximum positioning distance is determined by thedifference in the control potentials and the zero position by the sum ofthe potentials. As already explained above, the movement of the scanningneedle in the axial direction which occurs simultaneously on departingfrom the zero position and which can be read off from the two FIGS. 4a,b can in practice be neglected.

The potential of the electrode 8 is responsible for the movement of thescanning needle 19 in the axial direction, i.e. a movement perpendicularto the plane of the disk. Since the piezoelectric disk 5 is not fixed inthis region, a voltage change produces essentially a thickness change inthis region. The position of the tip of the needle changes under thesecircumstances by half the change in thickness as i$ illustrated by FIG.4c. As already explained above, the thickness of the disk is greater inthis field region than the length and width of the electrode 8. Thechange in length in the diameter of the disk associated with thethickness change due to the volume being constant is thereforenegligible in practice.

The movement of the scanning needle 19 in the radial direction, i.e. inthe direction of the longitudinal axis of the needle in the exemplaryembodiment, is achieved by changing the sum of the potentials of theelectrodes 6, 7 keeping the difference constant. As already mentioned,under these circumstances the piezoelectric disk 5 is in total stretchedor compressed in the direction of the disk surface. The change in lengthin the radial direction resulting from the volume being constant canonly have an effect in the direction of the open end of the jaws 1, 2because of the clamping of the disk 5. Because of the ratio between thedisk diameter and disk thickness, the superimposed movement in the axialdirection which can be read off from FIG. 4d is in practice negligible.

The electrode arrangement shown in FIG. 5 differs from that mentionedabove in that the electrodes 6', 7' corresponding to the two electrodes6, 7 are now formed as semi-circular areas. The electrode 8'corresponding to the small electrode 8 is integrated in the electrode17' corresponding to the electrode 17 and in this case carries thescanning needle 19. The movement functions assigned to the individualpotentials on these electrodes are the same as described above. Theorientation of the piezoelectric disk in the clamping device isanalogous. The simpler geometry of the electrode arrangement providescertain advantages during its production and in the mutual electricalinsulation of the electrode areas.

The above explanations have made it clear that the maximum adjustmentdistances in the three directions mentioned which are orthogonal to eachother depend not only on the absolute and relative values of thepotentials applied but also on the geometry of the piezoelectric disk.Assuming that the width and length of the small electrode 8 are about aslarge as the thickness of the piezoelectric disk 5, the followingapplies for the maximum thickness change d: ##EQU1## where r is theradius of the disk and h is its thickness. By choosing a suitablegeometry for the disk, the device can be adapted to the particularmeasurement task in relation to resonance frequency and maximumdeflection.

FIG. 6 shows a block circuit diagram for producing the necessarypotentials at the electrodes 6, 7, 8, 17 of the precision positioningdevice according to FIGS. 1, 2, 3. The voltages U_(rad), U_(tan), U_(ax)are proportional to the three space coordinates of the object to bepositioned and can be controlled independently. The electrode 6 has anadder connected to its input and the electrode 7 a subtracter. The basicpotential U_(rad) necessary for the radial movement is fed both to theadder and to the noninverting input of the subtracter as input voltage.The voltage U_(tan) responsible for the tangential movement is fed tothe adder and to the inverting input of the subtracter. The outputvoltages of the adder and the subtracter are in each case fed as acontrol voltage to a high-voltage amplifier 60, 70 whose outputssupplied (sic) the respective electrodes with the desired potentials. IfU_(rad) is constant, the sum of the potentials at the electrodes 6 and 7consequently remains constant, even if the difference in the potentialschanges as a result of alteration of U_(tan). The voltage U_(ax)necessary for the axial movement is likewise fed via a high-voltageamplifier 80 to the electrode 8 independently of the other voltages. Theelectrode 17 is connected via a high-voltage amplifier 170 to a constantpotential, which may also be the ground potential, in which case it ispossible to dispense with the high-voltage amplifier.

To scan an object--not shown--in a raster pattern with the scanningneedle 19, U_(tan) is expediently driven periodically linearly betweenits maximum values, U_(ax) being increased linearly at the same time.The value of U_(tan) then determines the length of a scanning line,while U_(ax) determines the spacing of the scanning lines for a halfcycle of the regulation of U_(tan) U_(ax) can also be increased in eachcase at the end of a scanning line abruptly by the value of one linespacing. U_(rad) determines the distance of the needle tip from theobject to be examined. This distance can be regulated via U_(rad) sothat a constant measuring signal is produced. However, U_(rad) can alsobe kept constant during a measurement, in which case the varyingmeasurement signal is recorded.

The regulation of the voltages necessary for the desired movement of thescanning needle can be controlled in a manner known per seadvantageously by a microprocessor. In this case, the superimposedmovements described above as negligible in practice can also becompensated for by suitable counter voltages acting in this direction.

The device according to the invention was tested with a piezoelectricdisk of approximately 10 mm diameter and 2 mm thickness consisting of amaterial known under the description PXE 5 (manufactured by Valvo) andsilver electrodes, the clamping device being manufactured from brass. Asthe object to be positioned use was made of a tungsten tip with which agold layer was examined under normal conditions and with oil immersionbetween tip and gold layer. Under these circumstances it was possible todetect clearly individual atoms in the gold layer, which corresponds toa lateral resolution of better than 3×10¹⁰ m.

We claim:
 1. A piezoelectric fine positioning device for moving anobject in three coordinate directions, comprising:a piezoelectric diskcut in a plane parallel manner; a layout of four flat electrodesarranged on the faces of the disk; electrical connections for applying apotential to each said electrode, thereby providing three independentlyadjustable electric fields in said disk; a fork-shaped clamping devicefor holding said disk at contact points symmetrically arranged aroundthe circumference of said disk, one said electrode positioned so as tolie outside of the area determined by the contact points of saidclamping device, said clamping device being electrically insulated fromsaid electrodes; and an object mounted on said disk, said objectpositioned within the electrical field region of said electrode lyingoutside of the area determined by the contact points of said clampingdevice.
 2. A piezoelectric fine positioning device according to claim 1,in which three electrodes are arranged on one face of said disk, theseelectrodes being separated and insulated from each other by a Y-shapeddividing means, and in which an undivided electrode is arranged on theother flat surface face of said disk.
 3. A piezoelectric finepositioning device according to claim 2, in which the electrode lying inthe V of the Y-shaped dividing means is substantially smaller than theother two electrodes on the same face of the piezoelectric disk, theother two electrodes being approximately equal in size to each other andsituated predominantly in the area by the contact points of thefork-shaped clamping device.
 4. A piezoelectric fine positioning deviceaccording to claim 1, in which one face of said piezoelectric disk isdivided into two semi-circular electrodes of equal surface area, and theother face of said disk is divided into a larger electrode and a smallerelectrode, said smaller electrode positioned on the opposite side of thedividing means between the two said equally sized electrodes, and anobject positioned on said smaller electrode.
 5. A piezoelectric finepositioning device according to claim I, in which said disk is round andthe contact points at the outer ends of the fork jaws are so chosen thatthe center point of the disk is situated inside the area determined bythe contact points.
 6. A piezoelectric fine positioning device accordingto claim 5, in which at least one third of the disk circumference ispositioned outside of the area defined by the contact points.
 7. Apiezoelectric fine positioning device according to claim 1, in which themaximum length or width of the smaller electrode is less than thethickness of the piezoelectric disk.
 8. A fine positioning deviceaccording to claim 1, wherein the electrical connections of theelectrodes are connected to a regulatable DC voltage source in a mannersuch that both the mean value of the potentials of two of the electrodesand the difference in their potentials, as well as the potential of theelectrode lying outside of the area determined by the contact points ofaid clamping device, are regulated independently of each other.
 9. Apiezoelectric fine positioning device according to claim 2, in whichsaid object is positioned on said undivided electrode.
 10. Apiezoelectric fine positioning device according to claim 5, in which atleast one third of the disk's circumference is positioned outside ofsaid forkshaped clamping device.
 11. A piezoelectric fine positioningdevice according to claim 1, in which the diameter of said disk isapproximately five times as great as its thickness.
 12. A piezoelectricfine positioning device according to claim 1, in which said deviceincludes electrical connections to each electrode, whereby an unequalpotential between said electrodes causes a responsive movement of theobject.